The present invention relates to radio communication, and more particularly to inductorless radio receiver and transmitter circuits.
In radio circuits of the prior art, inductive elements (coils, transformers, etc.) are commonly used in resonant filters, couplers, and other parts of radio tuners, oscillators, and amplifiers. It is desirable to avoid the use of inductors when miniaturization is sought because, at frequencies commonly encountered, the inductors are objectionably large. Indeed, although resistors, capacitors, and transistors are combined in increasingly dense integrated circuits, methods for producing inductors having appropriate characteristics in integrated circuits have not been devised.
Another problem with inductors in radio circuits is that they produce objectionable radiation. Such radiation can interfere with satisfactory operation of other portions of a system containing the inductors, and nearby equipment. Moreover, the electromagnetic radiation from the inductors of radio circuits can be detected and used for locating the circuit. This is a critical disadvantage in clandestine operations.
In tuning circuits it is also necessary to provide a high degree of selectivity, especially when many carriers are crowded within a narrow frequency band. Thus it is desired to have one or more tuned RF stages operating in conjunction with a variable local oscillator, the respective outputs being mixed for feeding an IF detector, one or more IF stages and a signal detector. Accordingly, it is often required to control a plurality of frequency determining elements in unison. In traditional radio circuits this was typically done by a ganged variable capacitor, each section thereof being connected in a separate frequency determining circuit. A disadvantage of such circuits is that the variable capacitors are quite bulky, in applications at conventional AM broadcast frequencies below about 1.6 Hz, and especially at low radio frequencies that extend below 250 KHz. When extreme miniaturization is sought, the bulk of variable reactance elements is a significant disadvantage even in the commercial FM band that extends up to about 125 MHz.
Voltage controlled oscillators (VCO's) are also used in radio receivers, thereby avoiding conventional tuning capacitors. However, these receivers usually do not include tuned RF stages because of the difficulties in tracking with the local oscillator frequency, and poor isolation between the control voltage and the various tuned circuits that would otherwise result; thus performance is often compromised.
Although tuned inductor circuits are commonly used in VCO's, these are bulky and produce objectionable radiation as noted above. Variable capacitance diodes (varactors) are sometimes used as the tuning element in VCO's, and can also be used to tune RF stages. However, these devices are best suited for higher frequency operation than the conventional AM broadcast frequencies of interest here. Also, the control voltage must be maintained relative to the potential of the tuned signal, with which there is limited isolation. Moreover, where a plurality of circuits are to be tuned in unison, they are difficult to match, and they may track poorly.
Thus there is a need for radio circuit apparatus for frequencies between 250 KHz and 125 MHz that is compatible in size with micro miniaturized integrated circuitry, that is easily and inexpensively produced, simple to tune, and does not produce objectionable radiation. This need is especially acute at frequencies below about 1.6 MHz.